FIG. 1 illustrates an aircraft 3 powered by counter-rotating propellers 6 and 9 of the ultra-high bypass, unducted fan type. The rotational directions are indicated by arrows 12 and 15. FIG. 2 illustrates a type of turbine system which can be used to drive the propellers 6 and 9.
In FIG. 2, fore propeller 6 is attached to a first turbine 18 which is supported by bearings 21 and 21A and rotates in direction 12 indicated in FIG. 1. The components rotating in this direction are decorated with hatching in FIG. 2. The aft propeller blade 9 is fastened to a second turbine 24 which is supported by bearings 27 and 27A, and rotates in direction 15 of FIG. 1. A hot, high-energy gas stream 30 provided by a gas generator (not shown) causes rotation of the turbines.
Propulsor blades 6 and 9 (which are sometimes called fans or propellers in the art) are of the variable pitch type. Variable pitch means that each blade can rotate about a respective pitch axis 6A or 9A in FIGS. 1 and 2 as indicated by circular arrows 33 and 36. A principal reason for changing pitch is to provide the blades 6 and 9 with the angle of attack which is proper for the present engine power level and airspeed of the aircraft.